Mobility is essential for human health. Regular physical activity helps prevent heart disease and stroke, relieves symptoms of depression, and promotes weight loss. Unfortunately, many conditions, such as cerebral palsy, osteoarthritis, and obesity, limit mobility at an enormous personal and societal cost. While vast amounts of data are available from hundreds of research labs and millions of smartphones, there is a dearth of methods for analyzing this massive, heterogeneous dataset. We propose to establish the National Center for Mobility Data Integration to Insight (the Mobilize Center) to overcome the data science challenges facing mobility big data and biomedical big data in general. Our preliminary work identified four bottlenecks in data science, which drive four Data Science Research Cores. The Cores include Biomechanical Modeling, Statistical Learning, Behavioral and Social Modeling, and Integrative Modeling and Prediction. Our Cores will produce novel methods to integrate diverse modeling modalities and gain insight from noisy, sparse, heterogeneous, and time-varying big data. Our data-sharing consortia, with clinical, research, and industry partners, will provide mobility data for over ten million people. Three Driving Biomedical Problems will focus and validate our data science research. The Mobilize Center will disseminate our novel data science tools to thousands of researchers and create a sustainable data-sharing consortia. We will train tens of thousands of scientists to use data science methods in biomedicine through our in-person and online educational programs. We will establish a cohesive, vibrant, and sustainable National Center through the leadership of an experienced executive team and will help unify the BD2K consortia through our Biomedical Computation Review publication and the Simtk.org resource portal. The Mobilize Center will lay the groundwork for the next generation of data science systems and revolutionize diagnosis and treatment for millions of people affected by limited mobility.